(a) Field of the Invention
The present invention is related to an antenna input circuit for an AM/FM radio receiver.
(b) Description of the Prior Art
With the conventional antenna input circuit for an AM/FM receiver, when receiving an AM broadcast, it is necessary to install an AM-receiving antenna in addition to an FM-receiving antenna and to connect the AM-receiving antenna to an antenna terminal provided exclusively for AM broadcast reception.
In FIG. 1 is shown an example of such a prior art antenna input circuit, which consists of an input transformer FMT for FM broadcast reception, an input transformer AMT for AM broadcast reception, antenna terminals T.sub.1, T.sub.2 and T.sub.4 connected as shown in FIG. 1, and a terminal T.sub.3 for making external grounding. The antenna terminal (jack) T.sub.1 is connectible with a co-axial cable, i.e. an unbalanced transmission line, leading from an FM-receiving antenna; the antenna terminal T.sub.2 is connectible with a twin-lead type feeder, i.e. a balanced transmission line, extending from an FM-receiving antenna; and the antenna terminal T.sub.4 is connectible with a transmission line leading from an AM-receiving antenna. One contact of the terminal T.sub.1 to which the shield sheath of the co-axial cable is to be connected is connected to the ground line of the antenna input circuit or the AM/FM receiver, and the other contact of the terminal T.sub.1 to which the center wire of the co-axial cable is to be connected is connected to a lead of the primary winding of the input transformer FMT. A pair of contacts of the antenna terminal T.sub.2 are connected to the corresponding leads of the primary winding of the input transformer FMT, respectively. The primary winding of the input transformer FMT has its center lead connected to the circuit ground line. The contact of the antenna terminal T.sub.4 is connected to a lead of the primary winding of the input transformer AMT, the other primary winding lead and the contact of the terminal T.sub.3 being connected to the circuit ground line, respectively.
As described above, the AM broadcast signal path and the AM-receiving antenna connection both are independent of those for FM broadcast reception, so that the prior art antenna input circuit has the inconvenience, in use, that installation and connection to the corresponding antenna terminals of both an FM-receiving antenna and an AM-receiving antenna is necessary.